Eight loop antenna system and method of scanning same



uly 1967 D. N. TRAVERS 3,329,954

EIGHT LOOP ANTENNA SYSTEM ANDMETHOD OF SCANNING SAME Filed Oct. 11, 1965 4 Sheets-Sheet 1 FIGZ.

INVENTOR. DOUGLAS IV. TRAVE'RS D. N. TRAVERS 3,329,954

ANTENNA SYSTEM AND METHOD OF SCANNiNG SAME 4 Sheets-Sheet 2 Jud 4, 1967 EIGHT LOOP Filed Oct. 11, 1965 FIG. 4

-25 35 2| -1 i- 6 FIG. 4a.

INVENTOR.

DOUGLAS N. TRAVERS July 4, 1967 EIGHT LOOP ANTENNA SYSTEM AND METHOD OF SCANNING SAME Filed Oct. ll 1965 D. N. TR

F w s m m ANTENNA SYSTEM C OSSED a LOOP H sIMPLE LOOPs 9b Cos. 6 Sin 9 Sin Cos ZBI I IMPEDANcE IMPEDANCE IMPEOANOE IMPEDANOE MATCHING MATCHING MATCHING MATCHING I I NETWORK NETWORK NETWORK NETWORK I 7 sIMPLE LOOP ,B 65 sPAcED LOOP 63-] GONIOMETER I GONIOMETER GEAR TRAIN AVERS 4 Sheets-Sheet 3 I05 I l II ,89 ,9I 93,

II EEPHASE SHIAFT IsOLATION IMZECDIAIIIEE GEAR TRAIN STATOR I: VARIABLE O IN AMP. NETWORK II 85 AND MIxER I II NETWORK I II5VAC I I D/F OUTPUT I I AN/sRD-7 RECEIVER I I I I I I RF a IF I 97 I AMPLIFIERS MODULATOR I I I 9 I'" F I I VIDEO 40 KC HORIZONTAL VERTICAL I DE E R OSCILLATOR AMPLIFIERS AMPLIFIERS I I I I I 99) vIDEO I I I I HASE I 7/ CONTROL I I (I23 I I I07 I l I .I I I J INVENTOR. DOUGLAS N. THAI/5R5 United States Patent 3,329,954 EIGHT LOOP ANTENNA SYSTEM AN METHOD OF SCANNING SAME Douglas N. Travers, San Antonio, Tex., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Oct. 11, 1965, Ser. No. 494,986 14 Claims. (Cl. 343-113) ABSTRACT OF THE DISCLOSURE A direction finding system having an array of two crossed-spaced-loop antennas spaced 45 apart in azimuth and having four loops each providing signals responsive to sin 20 and cos 20, 0 being the azimuth angle and connected to a twin channel or goniometer scanning and display system whereby bearing accuracy is increased and re-radiation eifects are minimized.

The present invention relates to an improved direction finding system and more, particularly, to a direction finding system incorporating an accurate and structurally strong antenna system having eight loops arranged in two four-loop pairs displaced 45 from each other, whereby in conjunction with appropriate scanning means, bearing accuracy is increased two-fold over prior art antennas and re-radiation effects are reduced to a minimum.

The present novel antenna system is comprised of two four-loop units, each loop of the resulting eight loops forming a side of an octagonal shape. Each unit has four loops displaced 90 from each other, each loop being arranged so that its plane is perpendicular to a radial extending thereto. This symmetrical antenna arrangement, when employed with a twin channel or goniometer scanning system, provides superior bearing accuracy with a minimum of re-radiation effects. Each of the antenna units produces one-half the observed bearing error for a given quality of beam pattern as compared with conventional crossed loops. An output from a channel as sociated with one four-loop unit provides a sin 20 output, called E herein, while the other four-loop unit provides a cos 20 or E output, 0 being the azimuth angle.

According to one version of the invention in which goniometer scanning is employed, there is connected to the balanced outputs of the two four-loop units, respective stators of a goniometer being driven at twice the.

speed of whatever sweep resolver presentation means are being used, for example, twice the speed of the receiver cathode ray tube 360 sweep generator. For ambiguity resolution, a desirable but not essential addition may comprise simple crossed loops connected to the stators of an additional goniometer driven at the sweep speed of the cathode ray tube. The output of this goniometer is shifted in phase by 90 and summed with the output of the spaced loop goniometer. In this manner, a threeloop pattern may be obtained which resolves possible four-way ambiguity of the spaced loop cathode ray tube display.

A twin channel scanning system may be used in place of the above-mentioned goniometer system, or, if desired, the twin channel and goniometer scanning systems may be used together to provide a combined display. Regardless of the display or scanning system employed with the present novel antenna system, the more essential aspect is that for vertical polarization the output E of one crossed spaced-loops is proportional to sin 26, and the output E of the other crossed spaced-loops is proportional to cosine 20. I

In addition to the advantage accruing to the present 'ice invention of providing presentation similar 'to that of a three loop system, that is, both sense and direction, the antenna of the present invention also provides for a Very sturdy, compact and reliable structural configuration Which may be easily placed upon a central upstanding support such as a mast or other protrusion. Of course, it is much more desirable to have fixed rather than rotating antennas on mast locations on ships. This advantage is provided by the antenna and system of the present invention without sacrifice of reliability or accuracy.

It is among the more salient objects of the present invention, therefore, to provide:

A novel, fixed direction-finding antenna system of high reliability and accuracy;

A novel fixed passive antenna including means for providing an output display having the advantages of a three-loop antenna; and

A novel direction finding antenna and scanning system for increased bearing accuracy and for increased reliability when mounted on an object such as a mast in a location having some radiation obstruction.

A feature of the present invention is the provision of additional antenna and a goniometer for use in conjunction with a novel antenna to provide a direction and sense display similar to that achieved with a three-loop antenna direction finding system.

A further feature of the present invention is the provision of a twin channel direction finding scanner in combination with the novel eight-loop direction finding antenna of the present invention.

The foregoing objects and features and advantages of the present invention as well as other advantages, features and objectives will be better understood by reference to the following detailed description of one specific embodiment thereof when taken in connection with the accompanying drawings in which like reference numerals are employed to represent like parts and in which:

FIG. 1 is a schematic view of a four-loop array and a spaced loop mode;

FIGS. 2 and 3 are views in schematic form of an equivalent eight-loop array of spaced loops according to the present invention for employment with a. scanning system such as a goniometer;

FIG. 4 is a view in perspective depicting structural details of an eight-loop spaced antenna made in accordance with the invention;

FIG. 4a is a schematic view of the antenna shown in FIG. 4 illustrating the antenna physically oriented about a ships mast;

FIG. 5 is a schematic block diagram of the system of the present invention employing a goniometer; and

FIG. 6 is a schematic block diagram of the system according to the invention employing a twin channel system.

A four-loop array in spaced-loop mode configuration 9a is shown in FIG. 1. The four-loop array of FIG. 1 may be considered as one-half of the complete eight-loop array according to the invention. A complete eight-loop array 9 is shown in FIG. 2 and consists of two identical, rigidly mounted four-loop antennas9a and 9b which are displaced 45 in azimuth from each other, the loop 9a being identical to the corresponding loop 9a shown in FIG. 1. The entire loop array in configuration resembles an octagon in plan view.

The antenna shown in FIG. 2 has two output terminals which are labeled E and E to correspond to the signals produced. The loops of the four-loop array associated with the terminal for B are labeled respectively 11a-d. Those loops of the second four-loop antenna array are given the reference numerals 13a-d. Each loop is balanced and shielded. For vertical polarization, the response of the antenna 11 at its terminal E is proportional to sine 20 and the response at the terminal E for the four-loop spaced antenna 13 is proportional to cosine 20, being the azimuth angle. It is appreciated that as long as the sine-cosine relationship as indicated above is maintained, any suitable receiving and indicating equipment may be adapted for bearing readout purposes by providing a readout display with a sweep rate of one-half the speed of the crossed spaced loop antenna scanning rate.

It is to be noted from FIGS. 1 and 2 that the opposing antenna loops each have a lead connected to one of its associated E or E terminals. In other words, the loops of each four-loop spaced antenna array may be considered to be connected parallel with respect to the terminals.

In the antenna arrangement shown in FIG. 3, in which the antenna lead connections are exactly the same as those shown in FIG. 2, each of the antenna loops may be in the form of a circle instead of a rectangle as shown in FIG. 1.

Reference is now made to FIG. 4 in which the antenna array, generally similar to the schematic shown in FIG. 2, is illustrated in structural detail. On an upstanding central support such as a radar mast 21 on board a ship, there is provided a mounting platform 23 attached or secured in any suitable manner (not shown) to the mast 21 so that the mast passes thru the center of said platform. Securely fastened in any suitable manner (not shown) to the top of the mounting platform 23 surrounding the mast or support 21 is a feed box 25 which contains the terminal receiver equipment for the antenna. The feed box 25 may conveniently take the form of an octagonal parallelepiped made of rigid material and upstandingly oriented on the mounting platform 23. The walls of the feed box 25 may be made of any suitable strong material such as metal, for example, type K rigid copper tubing.

The physical orientation of the loops about the feed box 25 and mast 21 may best be seen in FIG. 4a. Since each loop is identical in construction to each of the other loops, only one of the loops (shown in FIG. 4) need be described in detail, along with the attachments to its immediately adjacent loops. The loop to be described in detail is given the reference numeral 27, and its adjacent loops the reference numbers 27a and 27b, respectively. For supporting each of the identical loops, there is rigidly mounted a pair of transverse metalic shielding tubes 33 extending transversely outwardly from an anchoring base member 34 bolted or Welded to a wall of the feed box 25, the base member 34 shown in FIG. 4 as being bolted to feed box 25. Each pair of tubes 33 may be secured together in any suitable manner as by means of a plastic or metal band 35.

In construction, each of the loops such as loop 27 is shown as being made of metallic tubular material or tubes and as having a generally rectangular shape or configuration. This rectangular configuration is formed in part by opposing vertical legs or ends 37 and 39. The legs 37 and 39 are each comprised of two substantially identical tubular sections 37a-37b and 3941-3912 and the tubular sections are held in axial alignment and in rigid assembly by means of tubular T-joints 38. The tubular vertical ends or legs 37 and 39 are rigidly connected by suitable elbow joints 40 to an upper horizontal support tube 41, to an intermediate support tube 43, and to a lower horizontal support tube 45. There is thus formed by the foregoing tube components a substantially rectangular tubular frame member or frame having an intermediate horizontal support tube. The intermediate horizontal support tube 43 has rigidly attached to its central portion a hollow metallic mounting box 47. The hollow mounting box 47 is rigidly connected by any suitable rigid joint means (not shown) to the extremities of the pair of tubes 33 and is, therefore, rigid therewith. The intermediate support tube 43 is also rigidly connected to the mounting box 47 so that tubes 33 rigidly support the entire loop 27.

Each of the metallic frame components 37, 39, 41, 43 and 45 in this one embodiment of the invention is made of copper and constitutes an electrostatic shield for the antenna lead. In the central portion of the upper and lower tubes 41 and 45, there is disposed insulating gaps 49 and 51, respectively, which may be filled with any suitable insulating material. For example, a cylindrical phenolic insert may be employed for the gaps 49 and 51.

The antenna lead itself is comprised of insulated copper wire 52 which is passed continuously through each of the loops or frame components, the insulated wire 52 being indicated by the broken line in FIG. 4. In passing continuously through the frame components, the insulated wire 52 also extends substantially coaxially of each frame component.

The insulated wire encircles each of the frames or loops 27 with bore leads returning through one side of the horizontal support tube at the hollow mounting box 47 and then back to the feed box 25 at the center of the array. For example, a lead is passed coaxially through one of the tubes 33 via the left hand portion of the intermediate support tube 43, then upwardly through the upper portion of the vertical leg 37 via the upper horizontal tube 41, via tube 39, via tube 45, and upwardly through the lower portion of the tube 37 and the left hand portion of the intermediate horizontal tube 43 and back to the feed box 25 via the other of the tubes 33. The wire 52 may be rigidly supported for central location or axial extension within the shielding tubes by means of plastic or wooded inserts (not shown) or by any other suitable support means, the support means in each instance being made of course of a suitable insulating material.

Further structural strength is provided for the array by means of bands 53 and 55 connecting the respective upper and lower portions of the vertical tube supports 37 and 39 to similar portions of the adjacent or contiguous loops 27a and 27b, respectively. All of the tubular metallic material employed in the construction of the frame components above described has sturdy and rigid structural characteristics so that none of the tube elements exhibit distortion from secondary stresses. The result is an eight-loop array having essentially monolithic structural characteristics and which is rigid with the feed box and therefore is rigid with the platform 23 and mast 21.

In the construction or fabrication of the eight-loop array as shown in FIGS. 4 and 4A, the tubular frame components may be connected to the T-joints 38 and elbows 40 by using conventional joint welding techniques. The bands 53 and 55 may be of any suitable well-known design and, for the sake of illustration, may take the form of ordinary screw clamps made of metal. The portions of the loop such as at the vertical leg portions 37 and 39 may be in actual intimate physical contact with the vertical legs of the adjacent loops. It has been found that the employment of the metal screw clamps as well as the intimate physical contact of the conductive shielding material does not deleteriously affect the receiving characteristics of the loop antenna array.

It will be appreciated, of course, that the construction of the eight-loop array according to the present invention provides for symmetry and a very strong, rigid construction which is highly advantageous especially in mast location environments. It will be further appreciated that the antenna array configuration shown and described in FIGS. 4 and 4A enables the use of said array in through-mast configurations, that is, with the mast extending through the center of array so that additional equipment may be mounted on other locations of the mast both above and below the array.

Reference is now made to the block diagram of FIG. 5 showing the antenna array of the present invention employed in conjunction with a goniometer scanning system arranged to provide an improved direction-finding system. The crossed spaced loop antennas 9a and 9b are shown separately in FIG. 5 to clearly indicate their respective E (sin 20) and E (cos 20) outputs, although said antennas in the actual practice of the invention are arranged according to the configuration shown in FIG. 2. Outputs E and E are passed thru respective impedance matching networks 61 and 63 to respective stator windings 65 and 67 of a goniometer 69.

To resolve four-way ambiguities in the direction-finding system, there are provided two further fixed antennas 71 and 73 of well-known design arranged in conventional crossed simple loops to produce outputs cos 0 and sin 0, respectively. These outputs are passed via impedance matching networks to the respective stator windings 75 and 77 of a goniometer 79.

Goniometers 69 and 79 have rotors 81 and 83, respectively, which are driven by a variable-speed motor 85. A gear box or gear train 87 connected to motor 85 causes rotor 83 to be driven at twice the rotation rate as rotor 81.

The D/F output is produced by passing the output of rotor 81 via a 90 phase shifter 89 and isolation amplifier 91 to an impedance matching and mixer network 93. Network 93 also receives the output of goniometer 69 and consists of an impedance matching cathode follower and isolation amplifier before mixing with the output of amplifier 91. Network 93 then mixes the two signals with a broadband balun and provides a single D/F output. The output of network 93 is then passed to the input RF and IF amplifier and video detector stages of an AN/ SRD-7 receiver 95 (shown within broken line). Receiver 95 is of well-known design and the major components therefore are indicated by the blocks within the broken line. A modulator 97 located within receiver 95 receives as one input at suitable constant frequency signal, e.g., 40 kc., from an oscillator 98 and, as its other input, the video signal output of an AN/SRD-7 video detector 99. The output of modulator 97 is fed as an input to rotor windings 101 of a sweep resolver 103 of conventional well-known design and construction. Resolver 103 rotates in unison with goniometer 79 and therefore at one-half the speed of crossed spaced-loop goniometer 69, the speed of resolver 103 being reduced by gear box or gear train 105.

A display chassis 107, having standard well-known AN/SRD-7 components, has a horizontal sweep channel 109 with a conventional amplifier unit 111 and a vertical channel 113 with amplifier unit 115. A phase control circuit 117 insures synchronized relation between the signals in respective channels 109 and 113, said signals being produced by respective rotor windings 101 and a signal from modulator 97 corresponding to the position of a target detected by the antenna arrays 9, 71 and 73. A display is then produced on CRT 123 which is similar to that shown in FIGS. 4b, or 4d of Us. Patent 3,093,827 issued to the present inventor.

Referring now to FIG. 6 which shows in block diagram form, a twin channel eight-loop direction finder, the antenna units 9a and 9b produce respective outputs E (sin 20) and E (cos 20) which are fed via respective channels I and II to an impedance matching and calibration unit 151 (shown with broken lines). The unit 151 has a calibrating signal generator 153 providing a predetermined signal which may be fed via switches 155 and 157 into respective channels I and II.

In field operation, the detected signals are fed via switches 155, 157 to baluns 159 and 161 in respective channels I and II, said baluns (which may, for example, be cathode followers) providing balanced-to-unbalanced translation of the input signals.

The respective channel I and II outputs are fed as inputs to respective input terminals 163 and 165 of a twin channel receiver 167.

One type of receiver 167 which may be employed is the RACAL RA.153 receiver manufactured by Racal Electronics Ltd., of London, England. The Racal RA153 is a twin channel receiver designed primarily for high frequency D/F operation. The two separate signal paths maintain a close phase and gain balance over its entire frequency range of 1 to 30 mc.

Any suitable twin channel receiver unit 167 may, however, be employed. It is necessary in the twin channel receiver that each channel be as exactly matched in phase and gain as possible. Multiple superheterodyne conversion stages in each channel may be necessary to provide suitable frequency ranges for display purposes with known types of Oscilloscopes. For example, with a 1 to 30 mc. input frequency range, a kc. intermediate frequency may be suitable.

The outputs of twin channel receiver unit 167 are fed in channels I and II via a calibration oscilloscope 169 and, when employed with calibration signal generator 153, the channel I signal is phase and gain matched with that of channel II. The channel I and II signals are then fed to an X-Y oscilloscope 171, the channel II original signal being applied to vertical deflection plates and the channel I signal being applied to horizontal deflection plates. The potentials on the respective plates will determine the slope of a straight line rep-resenting an observed bearing, thus indicating the bearing of the target signals intercepted by perpendicular cross-spaced antenna array 9.

That bearings are observed on the CRT of oscilloscope 171 is borne out as follows:

If time varying plane waves (E'e are incident upon the antenna perpendicular to crossed spaced loops 9 and 9a, the following respective antenna-signals are obtained:

where E =h E' h =elfective height of the spaced loop 0=true bearing of incident radiation =phase difference, if any, between E and E If E, E respectively, are fed to the vertical and horizontal plates of the oscilloscope display tube by means of a phase and gain matched twin channel receiver, the bearing display is an ellipse. Where perpendicularly crossed spaced loops are provided as in the present invention, there being zero phase shift between channels, the display is a straight line whose slope is:

where 0 is the apparent bearing. Thus apparent bearing 6 will progress twice around the CRT for each 360-degree progression of the true bearing 0.

The straight line indication represented by the above equation is actually equivalent to four observed bearings, the opposite ends of the line giving the observed bearings 0 where:

These four observed bearings correspond to the four nulls observed on a CRT with a spinning spaced loop. Sensing means to resolve bearing ambiguity may be provided by simple crossed pair of loops in the same manner as described in conjunction with FIG. 5. Obviously, sensing means are not needed in those situations where two or more d/ f stations are employed together to determine the unique bearing of a target therefrom.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A direction finding system having fixed antenna array comprising:

a first four-loop unit having each loop located in a plane at a 90 increment about a centerpoint;

a second four-loop unit having each loop located in said plane at a 90 increment about said centerpoint, said second unit being displaced approximately 45 from said first loop unit;

each of said loops lying in a plane perpendicular to said plane and perpendicular to a radius emanating from said centerpoint; and

electrically conductive output means connected to said array for providing electrical signal output corresponding to a sine 20 function produced by one of said units and a cosine 20 function produced by the other of said units, being the azimuth of a source of electromagnetic wavefronts intercepted by said antenna array.

2. The system of claim 1 wherein said electrically conductive output means comprises scanning means responsive to the electrical signals produced by said array and display means having a predetermined sweep speed, and means driving said scanning means at a speed twice that of the sweep speed of said display means.

3. The system according to claim 1 including separate antenna means and electrical output means connected thereto for Sensing the direction of the source of said wavefronts, and electrical signal processing means coupling said electrical output means and said electrically conductive output means for providing further output signals containing information to resolve bearing ambiguity.

4. A ruggedized fixed antenna array for a direction finding system, said array being adaptable for being rigidly mounted about a central upstanding column as a support passing through, both above and below, said array, said array comprising:

a first four-loop unit having each loop located in a plane at a 90 increment about a centerpoint located in the column;

a second four-loop unit having each loop located in said plane at a 90 increment about said centerpoint, said second unit being displaced about 45 from said first unit;

each loop comprising a conductive lead and rigid tubular electrical shielding means surrounding said lead in spaced insulated relation thereto;

mounting means located on said column;

radially extending tubular means for each loop rigidly connected at inner portions to said mounting means and at outer portions to said rigid tubular electrical shielding means, whereby the conductive leads of each loop of the array may be passed from each of the loops via said radially extending tubular means and mounting means to a remote location; and

means rigidly connecting the adjacent tubular portions of the loops to each other.

5. The system of claim 2 wherein said scanning means comprises a goniometer system.

6. The system of claim 2 wherein said scanning means comprises a twin channel system.

7. The system of claim 3 wherein said electrical signal processing means includes a first goniometer for receiving on respective stator windings electrical output signals from said first and second four-loop units and a second goniometer for receiving signals from said separate antenna means;

means for driving the rotor winding of said first goniometer at twice the rate of said second goniometer;

electrical circuit means combining the outputs of the rotor windings of said first and second goniometers;

sweep resolving means having a rotating winding and first and second stator windings at right angles to each other;

means coupling the output of said electrical circuit means to the rotor of said sweep resolving means, said rotor being driven at the same rate as the rotor of said second goniometer;

5 and display means coupled to said first and second stators of said sweep resolving means and to said electrical circuit means for providing a non-ambiguous bearing indication of the source of intercepted signals.

8. A fixed antenna array for a direction finding system, said array being substantially octagonal in configuration and being carried by an upstanding column or the like extending therethrough, said array comprising:

platform means carried by said upstanding column;

a plurality of support members operatively connected to said platform means and extending therefrom substantially radially of said upstanding column and being angularly displaced approximately 45 from each other relative to the axis of said upstanding column;

a frame-like tubular shielding member carried by each of said tubular support members;

said tubular shielding members in combination defining a substantially octagonal configuration and each being positioned in a substantially vertical plane;

each of said tubular shielding members being substantially rectangular in shape and including a pair of vertical tubes, a pair of horizontal tubes and a horizontal intermediate tube;

said vertical tubes being connected to said horizontal tubes by elbow joints;

box means carried by each of said tubular supported members for connecting each of said tubular shielding members to each of said tubular support members;

means connecting each of said tubular shielding members to the tubular shielding member contiguous thereto;

conductive electrical leads carried within each of said tubular support members and tubular shielding members;

said conductive leads being electrically connected to provide a first four-loop antenna unit and a second four-loop antenna unit;

the four loops of each unit being displaced approximately 90 from each other and thus forming an eight-loop array with each loop being displaced approximately 45 from the loops contiguous thereto; and

insulating means carried within said tubular support members and said tubular shielding members for positioning and insulating said conductive electrical leads therein.

9. A fixed antenna array as claimed in claim 8 wherein the means for connecting each of said tubular shielding members to the tubular shielding member contiguous thereto takes the form of straps or bands.

10. A fixed antenna array as claimed in claim 8 wherein said platform means carries a feedbox and said plurality of tubular support members are mounted on said feedbox;

said tubular support members being mounted on the periphery of said feedbox at locations substantially 45 apart and extending substantially radially thereof.

11. A fixed antenna array for a direction finding system, said array being carried on a substantially vertically extending column or the like intermediate the ends of the column, said array comprising:

70 a first tubular unit having four tubular loops with the loops being angularly displaced approximately 90 from each other and each being positioned in a substantially vertical plane;

a second tubular unit having four tubular loops with the loops being angularly displaced approximately 9 90 from each other and being positioned in a substantially vertical plane;

the loops of said first and second units being angularly displaced approximately 45 from each other with the eight-loops being arranged in a substantially octagonal configuration;

each of said loops in said first and second tubular units being substantially identical in size and being substantially rectangular in configuration;

each of said tubular loops including pairs of vertical tubes, a pair of horizontal tubes and a horizontal intermediate tube, all of which are connected by tubular joints;

a tubular support member for connecting each tubular loop to said substantially vertically extending column;

said tubular support members extending substantially radially of said substantially vertically extending column and being angularly positioned relative to said substantially vertically extending column approximately 45 apart;

means rigidly connecting each of said tubular loops to the tubular 100p contiguous thereto;

and a conductive electrical lead carried within each tubular loop and being electrically connected to provide a first and second antenna unit each having four loops displaced approximately apart.

12. A fixed antenna array as claimed in claim 11 further including platform means carried by said substantially vertically extending column, box means mounted on said platform means with the walls of said box means being positioned around the periphery of said substantially vertically extending column;

one end of each of said tubular support members be ing rigidly connected to one of the walls of said box means.

13. A fixed antenna array as claimed in claim 12 wherein each of said tubular support members is comprised of a pair of tubes or tubular members and each tube is bolted to one of the walls of said box means.

14. A fixed antenna array as claimed in claim 11 wherein said tubular loops and said tubular support members are made of insulated copper wire.

References Cited Evans: IRE Transactions on Antennas and Propagation, vol. AP-lO, No. 6, November 1962, pp. 868-691.

RODNEY D. BENNETT, Primary Examiner. R. E. BERGER, Assistant Examiner. 

1. A DIRECTION FINDING SYSTEM HAVING FIXED ANTENNA ARRAY COMPRISING: A FIRST FOUR-LOOP UNIT HAVING EACH LOOP LOCATED IN A PLANE AT A 90* INCREMENT ABOUT A CENTERPOINT; A SECOND FOUR-LOOP UNIT HAVING EACH LOOP LOCATED IN SAID PLANE AT A 90* INCREMENT ABOUT SAID CENTERPOINT, SAID SECOND UNIT BEING DISPLACED APPROXIMATELY 45* FROM SAID FIRST LOOP UNIT; EACH OF SAID LOOPS LYING IN A PLANE PERPENDICULAR TO SAID PLANE AND PERPENDICULAR TO A RADIUS EMANATING FROM SAID CENTERPOINT; AND ELECTRICALLY CONDUCTIVE OUTPUT MEANS CONNECTED TO SAID ARRAY FOR PROVIDING ELECTRICAL SIGNAL OUTPUT CORRESPONDING TO A SINE 2$ FUNCTION PRODUCED BY ONE OF SAID UNITS AND A COSINE 2$ FUNCTION PRODUCED BY THE OTHER OF SAID UNITS, $ BEING THE AZIMUTH OF A SOURCE OF ELECTROMAGNETIC WAVEFRONTS INTERCEPTED BY SAID ANTENNA ARRAY.
 2. THE SYSTEM OF CLAIM 1 WHEREIN SAID ELECTRICALLY CONDUCTIVE OUTPUT MEANS COMPRISES SCANNING MEANS RESPONSIVE TO THE ELECTRICAL SIGNALS PRODUCED BY SAID ARRAY AND DISPLAY MEANS HAVING A PREDETERMINED SWEEP SPEED, AND MEANS DRIVING SAID SCANNING MEANS AT A SPEED TWICE THAT OF THE SWEEP OF SAID DISPLAY MEANS. 